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Consumer unit feed / main tails more than 3m

Jimi

Hi, what are the requirements when a consumer unit is to be located where the mains tails are more than 3m from the cut-out? Quickly searching online it seems that a switched fuse unit (60A, 80A, 100A as applicable) is required after the meter? Does anyone know the regulation number in BS7671? Some have mentioned taking high voltage drop and Ze into consideration for a long run. Is SWA usually used? It is safer and could be buried in a wall if needed couldn't it? Mains tails are a bit dodgy anyway as contacting the line you could get the full PFC so I prefer the idea of the earthed armouring. 25mm 3-core SWA at CEF is about £13 per meter. I guess that's what happens in blocks of flats?

In this installation, the current position of the intake and CU is in the kitchen. They would like it moving under the stairs if possible which would mean a 5 or 6 meter run from the kitchen, up above ceiling, across and down into the under-stairs cupboard. I could potentially take an alternative route behind the kitchen units. Thanks.

  • 0

    I am sure I replied to Geoff under his post starting, “could someone explain the reasoning….., but it has ended up under the post from Wallywombat! I have noted this happening before but I put it down to stupid old me! I think I will just go back to the quote method.

  • 0 in reply to lyledunn
    lyledunn said:
    Perhaps like the 6month RCD test, it’s just a number on the back of a fag packet!

    And the 5 s disconnection time.

    Yes, it's the same 'rule of thumb 3 m' as the 3 m distance we discuss in Regulation 433.2.2, i.e. where there is a reduction in CSA and where a supply-side protective device does not provide protection against overload current, we can place the device providing protection against overload current up to 3 m from the reduction in csa.

  • 0 in reply to gkenyon

    That is not the same situation as the meter tails.

    However, you appear to have confirmed my suspicion by misreading the regulation.

  • 0 in reply to lyledunn

    It's the forum. It does not work well.

    Z.

  • 0 in reply to geoffsd
    geoffsd said:
    That is not the same situation as the meter tails.

    Not sure whether you missed my point. This is why I said "it's the same" as the 3 m distance we discuss in Regulation 433.2.2.

    This particular rule of thumb being in actual fact a way of putting a quantitative value on "so close that we could perhaps consider the risk of fault is negligible, provided the wiring system is suitably installed to prevent faults, and preferably available to see there is no damage"

  • 0 in reply to gkenyon
    gkenyon said:
    Not sure whether you missed my point. This is why I said "it's the same" as the 3 m distance we discuss in Regulation 433.2.2.

    But it's not the same. Meter tails are not unprotected against overload nor are a reduction in CCC.

    Where 433.2.2 is appropriate, the 3m. rule only applies where fault current protection is inadequate and then the cable must also be "installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons".

  • 0 in reply to geoffsd

    Yes I think you've missed the point I was trying to make. I'm not saying it's not the same purpose (i.e. "Regulation"), but it's root of the "rule of thumb", i.e. for 433.2.2, and the DNO determining what they are wiling for their device to cover off, is the same effective installation requirement installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons

    So for 433.2.2, installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons equates to 3 m, and in the DNO requirement it equates to 3 m

  • 0 in reply to gkenyon
    gkenyon said:
    I'm not saying it's not the same purpose (i.e. "Regulation")

    So you are saying it is the same. It is not.

    My point is that the DNO cannot control what their fuse does merely because they say it will only work for 3m. so another identical one must be fitted near it.

    The tails (if compliant) are "covered off", if that means protected, by the DNO fuse against overload and fault current therefore the allowances of 433.2 (position of devices for protection against overload) and 433.3 (omission of devices for protection against overload) do not enter into it.

  • 0 in reply to Zoomup

    not like you to understate something Zoom

  • 0 in reply to geoffsd

      

    NO, I'm saying the "rule of thumb" is the "same rule of thumb", NOT that the requirements it's used to satisfy are the same.

    The rule of thumb being that if you only have 3 m of cable, the chances of a fault or damage are limited. That's all.

    You may want to interpret that I'm saying it's related to BS 7671 ... you'd be wrong there, although I accept the rule of thumb is used in cases there too

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